Magnetic resonance diffusion imaging promises a long-sought noninvasive method for mapping neural connectivity in vivo. However, the connectivity estimates derived from diffusion imaging have not been experimentally validated in any systematic way, nor has the method been applied to test any novel anatomic hypotheses. In order for the connectivity maps obtained from diffusion imaging to be interpreted with any reasonable confidence, it will be necessary to validate the technique against "gold-standard" tracer methods such as horse radish peroxidase histochemistry. In this project we propose to validate the connectivity maps obtained from diffusion imaging against the known cortical connectivity of the macaque monkey. The diffusion-based connectivity maps will exploit a novel diffusion imaging acquisition method called q-ball imaging, which is able to resolve complex intravoxel fiber crossings. The validation connectivity data set will be obtained from the CoCoMac neuroinformatics database that contains results from thousands of invasive tracer experiments in the macaque. This comparison will allow us to systematically evaluate the strengths and weaknesses of the diffusion method against a battery of classical histological techniques. We will then apply the optimized diffusion imaging method to test a series of hypotheses on the cortical connectivity of macaque visual system.